1. Field of the Invention
This invention relates to an optical tomography system for obtaining an optical tomographic image by measurement of OCT (optical coherence tomography).
2. Description of the Related Art
As a system for obtaining a tomographic image of an object of measurement in a body cavity, there has been known an ultrasonic tomography system. In addition to such an ultrasonic tomography system, there has been proposed an optical tomography system where an optical tomographic image is obtained on the basis of an interference of light by low coherence light. See, for instance, Japanese Unexamined Patent Publication No. 2003-172690. In the system disclosed in Japanese Unexamined Patent Publication No. 2003-172690, an optical tomographic image is obtained by measuring TD-OCT (time domain OCT) and the measuring light is guided into the body cavity by inserting a probe into the body cavity from the forceps port of an endoscope by way of a forceps channel.
More specifically, low coherence light emitted from a light source is divided into measuring light and reference light and the measuring light is projected onto the object of measurement, while the reflected light from the object of measurement is led to a multiplexing means. The reference light is led to the multiplexing means after its optical path length is changed. By the multiplexing means, the reflected light and the reference light are superposed one on another, and interference light due to the superposition is detected by, for instance, heterodyne detection. In the TD-OCT measurement, a phenomenon that interference light is detected when the optical path of the measuring light conforms to the optical path of the reference light in length is used and the measuring position (the depth of measurement) in the object is changed by changing the optical path length of the reference light.
When measuring the OCT by inserting a probe into a body cavity, the probe is mounted on the system body to be demountable since disinfection, cleaning and the like of the probe after use are necessary. That is, a plurality of probes are prepared for one optical tomography system and the probes are changed by the measurement. However there is an individual difference in the length of the optical fiber due to the manufacturing errors and the like, and the optical path length of the measuring light can change each time the probe is changed. Accordingly, in Japanese Unexamined Patent Publication No. 2003-172690, on the basis of the reflected light from the inner surface of a tube (sheath) covering an optical fiber of the probe, the optical path length of the reference light is adjusted to conform to the optical path length of the measuring light.
If the center of rotation of a probe is deviated from the center of a blood vessel when observing a tubular sample such as the blood vessel by rotating the probe, interference light from a part a tomographic image of which is unnecessary prevails to make the measurement inefficient. A method of preventing this has been proposed, for instance, in the U.S. Pat. No. 6,552,796. In the U.S. Pat. No. 6,552,796, the border is recognized from the tomographic images obtained, and the operator manually determines the offset of the center of measurement to store a storage portion a scanning initiating timing according to the amount of offset.
Whereas, as a system for rapidly obtaining a tomographic image without changing the optical path length of the reference light, there has been proposed an SS-OCT (swept source OCT) system where interference light is detected while the frequency of the light emitted from the light source is changed with time. In the SS-OCT system, an interferogram interference intensity signal is obtained without changing the optical path length by sweeping the frequency of the laser beam emitted from the light source to cause the reflected light and the reference light to interfere with each other by the use of a Michelson interferometer. Then a tomographic image is generated by carrying out a Fourier analysis on the interferogram signal in the region of an optical frequency.
Further, as a method of rapidly obtaining a tomographic image without sweeping the optical path length of the reference light, there has been proposed an SD-OCT (spectral domain OCT) measurement, In the SD-OCT measurement, a tomographic image is formed without scanning in the direction of depth, by emitting broad band, low coherence light from a light source by the use of a Michelson interferometer, dividing the low coherence light into measuring light and reference light and carrying out a Fourier analysis on each channeled spectrum signal obtained by decomposing the interference light of the reflected light, which returns when projecting the measuring light onto the object, and the reference light into frequency components.